Adenine phosphoribosyltransferase (APRT) catalyzes the reaction of adenine with 5-phosphoribosyl-1-pyrophosphate that produces adenosine monophosphate and pyrophosphate. In cases of complete APRT deficiency in man, adenine is oxidized by xanthine oxidase to form the highly insoluble and nephrotoxic derivative, 2,8-dihydroxyadenine (DHA). The accumulation of this compound may, in some instances, lead to life- threatening urolithiasis. APRT deficiency, which is inherited in an autosomal recessive manner, is a relatively rare inborn error of purine metabolism. However, about 1% of the population is heterozygous at APRT. Thus, far fewer APRT-deficient homozygotes have been observed than the frequency of heterozygotes would predict. We propose to: 1. Characterize mutant APRT alleles from additional APRT-deficient patients with regard to their DNA sequence and haplotype. Thus, we will investigate germline cell molecular mechanisms (mutations) that produce this disease. 2. Characterize APRT somatic mutations arising in vivo in T cells. Thus, we will determine whether or not T cell mutation, which is far easier to study than germline mutation, can be used as a model system for investigating mechanisms of germline mutagenesis. Somatic cell mutation is also important for understanding cancer. 3. Identify APRT heterozygotes by their reduced levels of red cell APRT activity and sequence their mutant alleles. Thus, we will determine whether or not the spectrum of mutant alleles in heterozygotes is different from that observed in enzyme-deficient homozygotes. This analysis may shed light on the apparent paucity of these homozygotes. 4. Produce a transgenic mouse model for human APRT deficiency. This animal model for the disease will be used for studies of underlying biochemical mechanisms and biochemical changes in general purine metabolism. For example, nucleotide pools, the disposition of metabolic adenine, and effects on de novo purine production will be studied. These animals will also provide a model system for autosomal somatic mutation that will complement the human T cell model.